This invention relates to a process for preparing such fine metal particles as copper particles, silver particles and silver-palladium composite particles.
Heretofore, a process for preparing fine copper particles has been employed which consists of reducing copper oxide particles in an aqueous medium by means of a reducing agent such as hydrazine. This process, however, has the disadvantages that since the particle size of the reduced particles depends on the particle size of the oxide particles, the control of the particle size is difficult to attain, and further since the reduced particles draw one another, only the aggregated copper particles having the broad particle size distribution can be obtained.
Moreover, in order to prepare fine silver particles a process has been employed which consists of forming silver oxide by adding sodium hydroxide to a silver nitrate solution, and reducing the silver oxide with formalin. This process, however, has the same disadvantages as those for the above process for preparing the copper particles.
In addition to the above single metal particles, composite metal particles containing two or more metals may be employed. In the electronics industry, electroconductive paste is employed for preparing a thick film circuit, most of which is mixed silver-palladium fine particle paste. The reason the palladium is incorporated into the silver-palladium paste is that migration and solder leaching can be avoided. For this purpose, several % to thirty % of the palladium should be incorporated.
Although the incorporation of the palladium provides considerable effects regarding the above two points, the method of mixing the fine particles of the silver and the palladium cannot uniformly disperse the palladium so that the above effects cannot be realized sufficiently.